Two Very Young Massive Stars Unshrouded with LGS AO

April 14, 2008

An international team led by Brazilian astronomer Cássio L. Barbosa
(UNIVAP, Sao José dos Campos) employed the Gemini North Laser Guide Star
(LGS) adaptive optics system Altair to probe deep into the core of star
forming region W51. This is one of the most luminous complexes of
massive star forming regions in our Galaxy.

W51 has two bright infrared sources: IRS1 and IRS2 which are coincident
with known radio sources. The team used the Near-Infrared Integral Field
Spectrograph (NIFS) to spectro-image three fields in IRS 2, which
contains three ultra-compact HII regions: W51d, d1 and d2 (Figure 1).
The spectral data cube they constructed allowed the researchers to
reconstruct spectral maps at various wavelengths, providing key
diagnostics on the composition and velocities of the stellar and gas
components in this obscure but active region. The high angular
resolution achieved by NIFS and Altair/LGS made possible the separation
of the strong extended emission from the massive young stellar objects
themselves.

IRS 2 is a site of intense star formation. It harbors some very massive
stars still embedded in dust due their young age. A reconstructed
spectrum of the ultra-compact HII region W51d/IRS2W shows a broad
Brackett gamma emission line of ~700 kilometers/second, betraying fast
winds coming from the hot ionizing star (Figure 2). The spectrum also
shows the NIII multiplet and an helium-II line in absorption. This
indicates a very early spectral type, as early as O3-4. Hence this star
may be one of the most massive ionizing sources of an ultra-compact HII
region known. The object is still buried in its opaque cocoon of dust.

For IRS2E, we seem to be seeing its photosphere. The detection of CO
overtone emission suggests the presence of an accretion disc around
IRS2E. The absence of radio emission and the accretion disk indicate
that the source is a massive young stellar object (Figure 3).

The NIFS observations have revealed two very massive stars, close in
space: one has cleared its surrounding material enough to probe its
photosphere; the other in a younger phase is still deeply embedded in
its dust cocoon and surrounded by a disk. This suggests that the
evolutionary timescale of such very young massive objects may be as
short as a few hundred thousand years.

For more details, see the article “High spatial resolution spectroscopy
of W51 IRS2E and IRS2W: two very massive young stars in early formation
stages”, by Cássio L. Barbosa et al. 2008, in the May 1st edition of
The Astrophysical Journal Letters, 678, L55

Technical Note:Using NIFS/Laser Guide Star AO

The authors of this accompanying work on W51 IRS2 used the Gemini North Laser Guide Star system feeding the facility adaptive optics system Altair, because no bright optical source was available for AO correction. A faint star was available for optical tip-tilt correction at the edge of the Altair field of view and this combination was successful.

Because the star forming region IRS2 in W51 is extremely rich in dust, extinction is an important issue. The stars in the region are embedded and appear as fuzzy blobs as revealed by radio mapping and mid-infrared imaging (see Figure 1). Using the 3 x 3 arcsecond aperture of NIFS ensured that spectral mapping was sufficiently extensive to catch the massive object embedded in dust cocoons. Narrow slits could have easily missed the core of the sources.

Figure 1. Composite image of W51 IRS2. The red contours are the radio
continuum emission at 2 cm, from VLA. The white contours represent the
mid-infrared emission at 12 microns as mapped by Gemini South T-ReCS.
The white squares correspond to the three fields observed with
NIFS/ALTAIR-LGS. The background image is a color composite image JHK
(BGR) and was obtained by VLT-NACO.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Maunakea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Científica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.